for National Geographic News
Offshore of Mexico's Yucatán Peninsula is a 110-mile-wide (180-kilometer-wide) crater dubbed Chicxulub, widely believed to be the site of an asteroid impact 65 million years ago that wiped out the dinosaurs (map of Mexico).
The mass extinction, thought to have obliterated two-thirds of the world's species in total, has been dubbed the K-T extinction because it bridges the Cretaceous and Tertiary geologic periods.
But a single asteroid impact doesn't tell the whole story, says a small but vocal group of geologists led by Princeton University paleontologist Gerta Keller in New Jersey.
Keller and her collaborators believe that the Chicxulub impact predated the K-T extinction by about 300,000 years.
The dinosaurs, they say, were killed not by a lone asteroid strike but by the quadruple whammy of global climate change, massive volcanism, and not one but two gigantic collisions. (Related: "Yucatán Asteroid Didn't Kill Dinosaurs, Study Says" [March 9, 2004].)
Peeling Back the Layers
The complex scientific detective story involves tiny glass beads, the rare element iridium, and sediments that might be deposits from gigantic tsunamis kicked up by the Chicxulub impact.
Both sides of the debate agree that the glass was created when the Chicxulub event filled the atmosphere with vaporized rock that quickly condensed and rained to Earth as tiny spherules, about a tenth of an inch (1 to 4 millimeters) in diameter.
The opposing sides also agree that the iridium came from an iridium-rich asteroid.
The problem is that the spherules and iridium occur in separate layers of earth, or strata, separated by as much as 25 feet (8 meters) of intervening sediment.
Conventional theory says that the intervening sediment was laid down quickly by a series of tsunami waves created by the Chicxulub strike.
But a detailed analysis of the sediment layers suggests that the sediment was formed between two separate asteroid strikes, one that laid down the glass and the other the iridium.
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